Replacing portions of a deteriorated or broken hip or shoulder has become an increasingly frequent operation that surgeons perform. One prosthesis that is commonly used replaces either a portion of the femur that connects with the pelvis or a portion of the humerus that attaches into the glenoid cavity of the shoulder. A variety of these prostheses have been developed, and they can be classified into one of three classifications that are based on the number of components that fit together to form the prosthesis.
The first classification of prosthetic devices has one solid component (excluding the head) that replaces a portion of the femur or humerus. This solid component can be viewed as having three sections, a stem section, a body section adjoining the stem section, and a neck section adjoining the body section. The stem section is an elongated section of the device that is inserted deeply into a channel reamed or machined into the bone to anchor the device in the femur or humerus. The stem section helps to prevent the device from rocking within the patient, and the stem section also spreads loads transmitted through the device over a larger area. The stem section may be straight, but many times the bone has a curvature or, as the bone is reamed or machined to receive the stem, the cutting device produces a curved channel into which the stem section must be inserted. Consequently, the stem section is often curved to fit either a curving bone or to fit wit a curving channel.
The second section of the solid-component implant is the body section. At least a portion of the body section fits within a cavity machined or broached into the end of the bone, where the bone widens. The body section is thus shaped to follow the widening shape of the bone, and the body section is also shaped to prevent the device from rotating within the bone in which the device is implanted. The body section also provides structural strength to the device as well as a large surface area for anchoring the device within the bone and for transmitting loads into the bone.
The third section of the solid-component implant is the neck section. The neck section adjoins the body section and provides an anchor point for the head at the correct angle and location to permit the head to properly engage the pelvis or the glenoid cavity.
The solid, one-component prosthesis described above enjoys wide-spread use. This prosthesis is used in roughly ninety percent of the hip implants currently performed in the U.S.
The second classification of prosthetic devices has two major components (excluding the head) that are fit together to form the device. One component of the two-component device embodies one of the three sections discussed above, and the other component embodies the other two sections. Thus, one component can be a combination of the stem and body sections, and its mating component is a neck component. Alternatively, one component can be a combination of the neck and body sections, and its mating component is a stem component. These devices are typified by those disclosed in U.S. Pats. No. 5,002,578, 5,002,581, 5,108,452, 5,135,529, 5,201,882, and 5,314,479. The devices disclosed in U.S. Pats. No. 5,080,685, 5,181,928, 5,286,260, and 5,370,706 are also considered two-component devices despite mention of three separate components in these patents, since the neck component described for those devices is no more than a screw used to hold the head to the body. In those devices, the angle and location of the head component are essentially determined by the screw-receiving bore within the body portion of the implant, making the body portion in effect a combined body/neck component.
Another classification of prosthetic devices has three major components (excluding the head). The device has a stem that inserts into the bone, a body that also inserts into the bone and into which the stem is mated, and a neck that mates to the body and to the head. One such device is disclosed in U.S. Pat. No. 5,002,578 (see especially FIG. 5).
Each of the three classifications of implants has problems or disadvantages associated with its design. The one-component device is usually forged from a blank or machined from a single block of metal. The blank used to forge the one-component device is costly, and if the one-component device is machined from a blank, much of the metal is lost during machining, and many complex and sequential machining steps must be performed to shape the finished device. The one-component device is, consequently, very expensive to manufacture. Also, because of the wide variety of shapes and sizes of femoral and humeral bones in the world's population, a distribution center or hospital must inventory a very large number of one-component devices having different dimensions in order to provide a prosthetic device that will fit a particular patient on whom surgery is being performed. However, to reduce inventory requirements, certain combinations of stem diameter and shape, body size and shape, and neck vertical and lateral offset are usually not supplied, which limits a surgeon's options in providing a prosthesis with a comfortable fit. If a hospital stocks sixty one-component devices, the surgeon has only sixty options from which to choose when fitting a patient with a prosthesis. Because of limited inventory, a surgeon can usually fit a patient with a prosthesis in which only one of the three sections (i.e. the head section, the neck section, is or the stem section) of the prosthesis fits well within the patient. The other two sections almost invariably are not ideally fit to the patient in whom the prosthesis is being implanted. Thus, with the one-component device, it is very difficult to have both a wide selection of fit and a reasonable inventory cost.
Further, precise installation of a single-component device within a patient can be very difficult, especially for a device in which the stem section is fluted to better anchor the device and prevent it from rotating. Once the flutes begin to encounter bone, the flutes cut their own channels to help anchor the implant. If the surgeon discovers that the implant does not fit as well as the surgeon wishes, the surgeon must dislodge the implant from the bone (causing further trauma) and either reposition the implant or remove the implant and substitute one having the appropriate dimensions before seating the implant once again. Despite these disadvantages, the one-component device remains the most widely-used implant today.
The two-component device also has a number of problems or disadvantages associated with its design. Like the one-component device, a component such as a combined neck-and-body component or a combined body-and-stem component typically requires many machining steps and is thus costly to make. Also, because of the wide variety of shapes and sizes of femoral and humeral bones in the population, a large number of these components must be inventoried by a hospital, or the hospital must select only certain sizes and compromise the fit of the device within the patient. An inventory of one hundred thirty pieces provides only approximately two hundred surgical options. At best, the surgeon can usually provide a good fit within a patient for two of the three (neck, body, and stem) sections.
Precise placement of the two-component device within a patient can also be very difficult where the body and neck sections or the stem and body sections are combined in one component, and especially where the stem is fluted and/or the channel in which the stem section is inserted is curved. A surgeon may need to remove and reinsert components similarly to the one-component device described above, causing additional trauma to the patient. Although the magnitude of these problems or disadvantages is somewhat reduced when compared to the problems associated with the one-component device, the magnitude of these problems remains significantly high for the two-component device.
The three-component device disclosed in U.S. Pat. No. 5,002,578 also has disadvantages associated with its design. The stem component and the body component must be joined to form a sub-assembly prior to implanting this sub-assembly within the patient. It is difficult to precisely align the stem component and body component outside of the patient's body so that a body component and a curved stem component have the correct relationship to one another to fit within the channel and the cavity formed in the bone, and it is also difficult to precisely position a sub-assembly during insertion into the bone to assure that the sub-assembly is accurately positioned within the bone as flutes on the stem cut into the bone and anchor the sub-assembly.
Thus, various devices discussed above have a number of disadvantages. A large number of parts must be stocked, which creates high inventory costs for hospitals. This also requires the surgeon to take precious time during an operation to select the correct component(s) from the large number in stock. The particular dimensions to which the device must be fit are not known until the patient has undergone surgery to remove part of the deteriorated bone and until the bone has been trimmed and reamed or broached to accept the stem and body of the device. Consequently, selection of components and their assembly is performed during surgery, and any time needed to select components, assemble, and adjust the device lengthens the time that the patient is undergoing surgery. Further, in an effort to control inventory costs, a hospital or company supplying the components may limit the number of components available, which forces the surgeon to compromise the fit and comfort of the prosthesis for many patients.
The parts are usually costly, since they require lengthy and complex machining procedures to manufacture them. The parts are also not easily adjusted when being installed into a patient. A slight misalignment of a component because of incorrect assembly causes the whole assembly to be very difficult to install. A slight misalignment is not known until the surgeon attempts to install the device in the patient. If the stem is attached to the body and is slightly misaligned so that it causes the body and neck not to fit in the patient properly, the assembled device must be removed from the patient, any bond between the stem and the body components must be broken, and the stem must be adjusted and rebonded to the body. The assembled device is reinserted into the patient, and if the stem is not yet properly aligned, the device must again be removed from the patient and the process repeated until the components in the device are properly aligned. It would clearly be advantageous if prostheses could be made having a wide variety of dimensions from few components that require little machining. It would further be clearly advantageous if prostheses could be made that are easily adjusted during surgery while installing these prostheses within patients.
It is an object of certain embodiments of this invention to provide a modular assembly having three major components that are easily and independently adjustable during their assembly into a patient. It is another object of certain embodiments of the invention to provide a three-component modular assembly in which the body and neck components individually can be rotated around the axis of the stem component to independently adjust their version angles during assembly of the prosthetic device. It is a further object of certain embodiments of the invention to provide a three-component modular assembly that is self-tightening. It is another object of certain embodiments of this invention to provide a modular assembly that can be quickly installed. It is another object of certain embodiments of this invention to provide a smaller set of components from which a surgeon will choose without reducing the number of combinations of fit of components, so that the surgeon can provide a patient with the combination of components that fit the patient's particular bone structures without requiring high inventory levels of components. It is another object of certain embodiments of this invention to provide an assembly of three components, each of which requires few machining steps to manufacture. Further objects and advantages of the invention are apparent from the discussion herein.